1. Field of the Invention
The present invention relates to a plane antenna which is installed in a horizontal portion such as the roof, trunk lid, etc. of an automobile.
2. Prior Art
Antennas which show little projection are in demand as antennas for mobile units such as automobiles, etc. in order to eliminate wind noise and to improve the external appearance of the vehicle. From this standpoint, plane or flat plate form antennas are desirable.
One type of plane antenna is a slot antenna. This is an antenna in which a portion of the conductor is cut out in the form of a circle (generally, cut-out slots are formed at regular intervals to form such a circle).
In conventional plane antennas, the mean diameter of the central conductor, i.e., the island-form conductor in the center, reaches approximately 1 m in the case of FM reception, so that the length of the circle (i.e. the circumference) is about 1 wavelength, that is, 3.3 m when the wave received has a frequency of 90 MHz.
Accordingly, when an attempt is made to install another plane antenna whose frequency (e.g. the 900 MHz band used for automobile telephones) differs from that of the first plane antenna installed (e.g., the 90 MHz band used for FM reception), and it is attempted to accomplish this antenna installation with the loss of any performance, such installation is very difficult in areas where the amount of space available is limited, as in the roof of a vehicle, etc.